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Re: Documenting MS_LAZYTIME

To: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
Subject: Re: Documenting MS_LAZYTIME
From: Andreas Dilger <adilger@xxxxxxxxx>
Date: Fri, 20 Feb 2015 05:32:58 -0700
Cc: Theodore Ts'o <tytso@xxxxxxx>, Ext4 Developers List <linux-ext4@xxxxxxxxxxxxxxx>, Linux btrfs Developers List <linux-btrfs@xxxxxxxxxxxxxxx>, XFS Developers <xfs@xxxxxxxxxxx>, linux-man@xxxxxxxxxxxxxxx, Linux-Fsdevel <linux-fsdevel@xxxxxxxxxxxxxxx>, Linux API <linux-api@xxxxxxxxxxxxxxx>
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On Feb 20, 2015, at 1:50 AM, Michael Kerrisk <mtk.manpages@xxxxxxxxx> wrote:
> 
> Hello Ted,
> 
> Based on your commit message 0ae45f63d4e, I I wrote the documentation
> below for MS_LAZYTIME, to go into the mount(2) man page. Could you
> please check it over and let me know if it's accurate. In particular,
> I added pieces marked with "*" below that were not part of the commit
> message and I'd like confirmation that they're accurate.
> 
> Thanks,
> 
> Michael
> 
> [[
>       MS_LAZYTIME (since Linux 3.20)
>              Only  update  filetimes (atime, mtime, ctime) on the in-
>              memory version of the file  inode.   The  on-disk  timeâ
>              stamps are updated only when:
> 
>              (a)  the inode needs to be updated for some change unreâ
>                   lated to file timestamps;
> 
>              (b)  the application  employs  fsync(2),  syncfs(2),  or
>                   sync(2);
> 
>              (c)  an undeleted inode is evicted from memory; or
> 
> *             (d)  more than 24 hours have passed since the i-node was
> *                  written to disk.
> 
>              This mount option significantly reduces  writes  to  the
>              inode  table  for workloads that perform frequent random
>              writes to preallocated files.
> 
> *             As at Linux 3.20, this option is supported only on ext4.

I _think_ that the lazytime mount option is generic for all filesystems.
I believe ext4 has an extra optimization for it, but that's it.

Cheers, Andreas





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